The present invention relates to interior automotive parts formed from polymeric blends and more specifically it relates to blends of poly(alkylene terephthalate). Even more particularly, the present invention relates to a compatibilized blend of poly(butylene terephthalate) and at least three block segments which alternate between poly(butylene terephthalate) and poly(alkylene terephthalate) where the alkylene used is more than four carbons in length, along with a non-volatile solid low molecular weight copolymer salt compatibilizer.
Copolyesters are well-known commodity thermoplastic elastomers. They have been extensively used in applications where high strength and high elongation are required such as, for example, constant velocity joint boots in front-wheel-drive axles. Because copolyesters are very flexible, having a flexural modulus less than 350 Megapascals, many uses for this material require blending with a polyester to make the material stiffer.
Generally, when the amount of polyester used to stiffen the copolyester is greater than 500 Megapascals, some inhomogeneity appears in the blend. This inhomogeneity causes inconsistent performance in molded automotive parts. The location of the inhomogeneity zone in the molded part moves around randomly from part to part. Whenever the zone moves into a high stress concentration area within the part, fracture may occur because the mechanical properties in the inhomogeneity zone are inferior.
Compatibilizers for polyester and copolyester blends have not been developed in the past because when either component is in very small quantities (below the saturation point of dispersion), the blend is homogeneous.
Briefly, an essential problem in the past with the use of copolyester blends is that homogeneity was achieved by limiting to a blend that was either slightly stiffer than a copolyester or slightly more flexible than a polyester. In addition, the final composition should not rely for its performance characteristics on any components that are at all volatile, as the long periods of high temperature to which an automotive interior can be exposed will cause such components to volatilize out of the plastic.
Thus, from the foregoing, it will be seen that there is a need in the art to provide for homogeneous copolyester blends that cover the whole spectrum of stiffness with a copolyester being at the flexible extreme and a polyester at the stiff extreme. Such formulations could then be used, for example, as a stiff air bag door which would match the stiffness of the instrument panel of a full-size pickup truck, needed to withstand road vibrations. Finally, there is a need in the art to provide for a material which can compatibilize copolyester blends for homogeneity and which is non-volatile.
Thus, in accordance with this invention, copolyester blends are compatibilized by use of a low molecular weight compatibilizer. This compatibilizer is non-volatile and accordingly is not subject to exudation, evaporation, and condensation onto adjacent areas like interior windows and parts of an automobile. When molded, the compatibilized copolyester blend is capable of molding at a 13xc2x0 C. (24xc2x0 F.) lower melt temperature, making it especially beneficial for heat aging resistance.
Surprisingly and unexpectedly, and unlike other low molecular weight additives which have been added to polymer blends, the use of the present compatibilizer, instead of increasing the melt index (lowering the melt viscosity) of the copolyester blend, decreases it. The present compatibilizer is capable of increasing the hardness of the copolyester blend from about 52 Shore D to about 58-60 Shore D, even though the compatibilizer itself is softer than the copolyester blend alone.
Thus, in accordance with the present invention, a composition of matter is disclosed which comprises a blend of a polyester such as poly(butylene terephthalate), a copolyester such as segmented poly(alkylene terephthalate), and a low molecular weight polyvalent copolymer salt having a melting point not greater than 95xc2x0 C. Generally, the copolymer salt will be employed in an amount effective to lower the melt index by at least 13%. The amount of copolymer generally will be more than 0.1% by weight and, typically, in the range of about 0.1% to about 10%.
The polyester used in the practice of the invention may be any of a number of well-known polyesters prepared by condensation polymerization of a polyacid (e.g., adipic acid or anhydride, oxalic acid or anhydride, phthalic acid or anhydride, maleic acid or anhydride, or terephthalic acid or anhydride) with a polyol (e.g., ethylene glycol, propylene glycol, 1,6-hexane diol, and the like). In a preferred embodiment, the polyester is a polyalkylene terephthalate, and more preferably is poly(butylene terephthalate) (PBT). The copolyester is prepared by the condensation reaction of a polyacid and a polyol where at least one of these components is an already-formed polyester, optionally in combination with monomeric polyacids and or polyols to form a copolyester such as segmented poly(alkylene terephthalate) (PAT), as is known in the art. The low molecular weight (LMW) polyvalent copolymer salt may be prepared by first copolymerizing one or more addition-polymerizable monomers including at least one nonionic monomer (e.g., an olefinic monomer such as ethylene or propylene) and at least one saltable ionic monomer (e.g., an acid such as acrylic acid or methacrylic acid) to form a copolymer having a number average molecular weight that is preferably less than 5000. After polymerization, the copolymer is then salted with a polyvalent metal ion (e.g., calcium, barium, magnesium, antimony, and the like). The three materials discussed above may be blended in any conventional and convenient manner, as a solution, a latex, or as a melt as, for example, in a twin-screw extruder.
The amount of comonomers in forming the segmented copolyester may vary so long as the copolyester behaves like a thermoplastic elastomer. In one preferred embodiment, the segmented copolyester contains (1) about 5-90 weight percent long chain ester units derived from at least one long chain glycol having a molecular weight of about 600-6000, and at least one low molecular weight dicarboxylic acid having a molecular weight less than about 300, and (2) about 10-95 weight percent short chain ester units derived from at least one low molecular weight diol having a molecular weight of less than 250 and at least one low molecular weight dicarboxylic acid having a molecular weight of less than about 300. The amount of olefin, acid, and polyvalent metal ion employed in forming the low molecular weight copolymer salt may vary from about 10 to about 180 acid number by titration along with about 5% to about 100% neutralization as long as the compatibilizer homogenizes the polyester and copolyester blend.